Electric motor having electrostatic shield arrangement

ABSTRACT

An electromechanical machine includes a stator fixed with respect to a housing structure and a rotor fixed with respect to a driven shaft. The stator includes a magnetically permeable core having a plurality of parallel winding slots containing conductive windings. Coilheads are located at opposite axial ends of the magnetically permeable core where the windings turn to extend down a parallel winding slot. The motor is equipped with an electrostatic shield arrangement to reduce capacitive coupling between the stator and rotor during operation. The shield arrangement comprises a grounded conductive layer separated from the conductive windings of the stator by an insulative layer of cured resin material. Preferably, the conductive layer may be formed of a conductive paint.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 09/008,043, filed Jan.16, 1998, now U.S. Pat. No. 6,202,285.

BACKGROUND OF THE INVENTION

The present invention relates generally to the art of electric motorsand other electromechanical machines. More particularly, the inventionrelates to an improved electrostatic shield arrangement for use in anelectromechanical machine.

The shaft of an AC induction motor or other electromechanical machine isoften supported by bearing assemblies maintained in position by themachine housing. In one known construction, each bearing assembly issupported by a portion of the housing known as the “end bell.” As itsname implies, the end bell is located at one axial end of the housing,and defines a hole through which the rotatable shaft freely extends.

During operation of an electric motor, capacitive coupling can oftenoccur between the stator and rotor. Occasionally, the potentialdifference developed in this manner will exceed a magnitude necessary tobreak down insulating grease in the bearing assembly. In this case,currents may arc or discharge from the bearing balls or rollers to theouter bearing race causing “pits” or other undesirable effects. As aresult, more frequent servicing of the bearing assemblies may berequired.

The prior art has provided electrostatic shield arrangements to reducecapacitive coupling between the rotor and stator, and consequent currentdischarge through the bearing assemblies. Examples of various shieldconfigurations can be seen in U.S. Pat. No. 5,661,353 to Erdman et al.,incorporated herein by reference. While these arrangements have beeneffective at reducing capacitive coupling, a further need exists forvarious novel electrostatic shield arrangements that are compatible withmass production techniques.

SUMMARY OF THE INVENTION

The present invention recognizes and addresses the foregoingdisadvantages, and others of prior art constructions and methods.Accordingly, it is an object of the present invention to provide anelectromechanical machine having a novel electrostatic shieldarrangement.

It is a more particular object of the present invention to provide anelectromechanical machine having a novel electrostatic shieldarrangement that is compatible with mass production techniques.

It is a further object of the present invention to provide a novelelectrostatic shield arrangement that is compact and effective.

It is a further object of the present invention to provide a novelelectrostatic shield arrangement in which a grounded conductive layer isseparated from stator windings by an insulative layer.

It is also an object of the present invention to provide a novel methodof manufacturing a stator for use in an electromechanical machine.

Some of these objects are achieved by an electromechanical machinecomprising a housing structure rotatably supporting a shaft along apredetermined central axis. A stator having a plurality of conductivewindings radially spaced about the central axis is fixed with respect tothe housing structure. A rotor is also provided, located radially inwardof the stator and fixed with respect to the shaft.

The electromechanical machine further comprises an electrostatic shieldarrangement attached to the stator and adapted to interpose theconductive windings and the rotor. The shield arrangement comprises aconductive layer separated from the conductive windings by an insulativelayer of resin material. Preferably, the insulative layer of the shieldarrangement is located in respective winding slots defined in amagnetically permeable core of the stator. The conductive layerpreferably comprises a nonmagnetic conductive material located radiallyinward of the insulative layer in the respective winding slots. Theconductive layer may be in electrical communication with themagnetically permeable core, and grounded thereby. The stator alsoincludes first and second coilheads located at opposite axial ends ofthe magnetically permeable core. In such embodiments, the shieldarrangement is further located on an inner surface of the first andsecond coilheads. Preferably, the conductive layer of the shieldarrangement is continuously conductive along an axial extent from thefirst coilhead across the slot windings to the second coilhead.

In exemplary embodiments, the insulative layer of the shield arrangementcomprises a cured resin, such as a glass-filled resin, applied to theconductive windings to a predetermined thickness. Preferably, the resinsubstantially entirely impregnates the conductive windings of thestator. Furthermore, the conductive layer of the shield arrangement maycomprise a metallic paint applied to a surface of the resin. Aprotective top coat may also be applied over the conductive layer on aninner surface of the stator.

Other objects of the present invention are achieved by anelectromechanical machine comprising a fixed stator having conductivewindings located in a plurality of parallel, axially extending windingslots defined about an inner surface of a magnetically permeable core.The stator further comprises first and second coilheads located atopposite axial ends of the magnetically permeable core. A movable rotoris located radially inward of the stator.

The electromechanical machine further includes an electrostatic shieldarrangement attached to the stator. The shield arrangement includes aconductive layer separated from the windings by an insulating layer ofresin material. The shield arrangement is located in the winding slotsas well as on an inside surface of the first and second coilheads tointerpose the conductive windings and the rotor.

The conductive layer of the shield arrangement preferably comprises anonmagnetic conductive material located radially inward of theinsulative layer. Preferably, the conductive layer is in electricalcommunication with the magnetically permeable core and is groundedthereby.

Other objects of the present invention are achieved by anelectromechanical machine comprising a fixed stator having conductivewindings located in a plurality of parallel, axially-extending windingslots defined in a magnetically permeable core. A movable rotor isoperative to have a magnetic flux induced therein by excitation of theconductive windings of the stator. The conductive windings have a curedresin applied thereto to yield an insulative layer of predeterminedthickness between the conductive windings and the rotor. A conductivelayer of metallic paint is applied to the insulative layer and therebyseparated from the conductive windings. The insulative layer and theconductive layer thus form an electrostatic shield arrangementinterposing the conductive windings and the rotor.

Other objects of the present invention are achieved by a method ofmanufacturing a stator for use in an electromechanical machine. One stepof the method involves providing a magnetically permeable core havingconductive windings located in a plurality of parallel,axially-extending winding slots. An uncured resin material is applied tothe conductive windings in a sufficient amount to yield an insulativelayer of predetermined thickness. Next, the resin material is at leastpartially cured. A metallic material is then applied over the insulativelayer to form a conductive layer in electrical communication with themagnetically permeable core.

According to exemplary methodology, the resin material is only partiallycured prior to the metallic material being applied to the insulativelayer. In such cases, the resin material is finish cured after themetallic material has been applied to the insulative layer.

Often, it will be desirable to apply and partially cure the resinmaterial more than once before the metallic material is applied to theinsulative layer in order to achieve the predetermined thicknessthereof.

Other objects, features and aspects of the present invention areachieved by various combinations and subcombinations of the disclosedelements, which are discussed in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, to one of ordinary skill in the art, is set forthmore particularly in the remainder of the specification, includingreference to the accompanying drawings, in which:

FIG. 1 is an elevational view of an electric motor showing the motorhousing partially cut away to reveal various internal componentstherein;

FIG. 2 is a perspective view of the stator and a portion of the motorshaft, with the motor housing being shown in phantom;

FIG. 3 is an enlarged partial cross-sectional view as taken along line3—3 of FIG. 2;

FIG. 4 is an enlarged partial cross-sectional view of a stator coilheadshowing the electrostatic shield arrangement located thereon;

FIG. 5 is a further enlarged view of the area so indicated in FIG. 4;and

FIG. 6 is a flow diagram showing steps of producing a stator for use inan electromechanical machine according to the present invention.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstructions.

Referring now to FIGS. 1 and 2, an electric motor 10 constructed inaccordance with the present invention has a rotatable shaft 12 extendingalong a central axis. The internal components of motor 10 are enclosedby a housing including a main housing portion 14. One or more eyebolts16 may be provided to facilitate lifting of motor 10. Main housingportion 14 defines an appropriate base 18 on which motor 10 will restduring use.

The housing of motor 10 further includes end portions, such as end bell20, located at respective axial sides of main housing portion 14. Theend portions may be attached to main housing portion 14 by anyappropriate means, such as by bolts. Typically, each end portion willmaintain a respective bearing assembly, such as bearing assembly 22, tofacilitate rotation of shaft 12.

Shaft 12 continues through bearing assembly 22 and beyond end bell 20for connection to other equipment. The opposite end of shaft 12 carriesa fan 24, located within shroud 26. Due to the configuration of shroud26, rotation of fan 24 causes cooling air to circulate around variouscooling fins 28 defined on the exterior of main housing portion 14.

Inside of its housing, motor 10 includes a stator 30 that remains fixedduring operation. Stator 30 includes a magnetically permeable core 32preferably comprising a plurality of relatively thin laminationsarranged in a stack. As indicated at 34, longitudinal windings arelocated in parallel, axially-extending slots defined about the insidesurface of core 32 to provide a flow path for flux-generating current.The windings turn at respective coilheads 36 and 38 to return along aparallel slot.

A rotor 40, secured to shaft 12, desirably rotates based on theelectromagnetic interaction between it and stator 30. In the illustratedembodiment, motor 10 is an induction motor, wherein rotor 40 isconstructed as a “squirrel cage” in a known manner. A plurality ofradial vanes, such as vanes 42, may be provided at the periphery of therotor ends to circulate cooling air inside the motor housing.

During operation, capacitive coupling between the stator and rotor of anelectric motor may produce errant currents through the bearingassemblies utilized to support the rotatable shaft. As described above,arcing in the bearing assembly can pit the race on which the ball orroller bearings ride, leading to increased maintenance requirements. Toreduce capacitive coupling, the present invention provides anelectrostatic shield arrangement, indicated generally at 43, interposedbetween rotor 40 and the conductive windings of stator 30.

The electrostatic shield arrangement provides a conductive path toground for charge that could otherwise collect on rotor 40. In addition,the electrostatic shield arrangement is constructed so that theconductive path will be well-insulated from the conductive windings ofstator 30. This is particularly advantageous in inverter-driven motorapplications due to the high switching voltages that can be developed.

Referring now to FIGS. 3 and 4, the electrostatic shield arrangement asutilized in the illustrated embodiment can be most easily explained. Inparticular, FIG. 3 shows the location of the shield arrangement in thearea of stator 30 axially between coilheads 36 and 38. The constructionof the shield arrangement on the coilheads may be understood withreference to FIG. 4, where only coilhead 36 is shown for purposes ofexplanation.

As can be seen in FIG. 3, conductive windings 34 each comprise aplurality of individual conductors situated in parallel,axially-extending winding slots, such as slot 44. Although theconductors are individually insulated, an insulative slot liner 46 ispreferably located in each slot to further prevent the possibility ofgrounding to core 32. An insulative top liner 48, known as a “topstick,” may be located at the “top” of each slot as shown.

Looking now also at FIG. 4, the electrostatic shield arrangementincludes an insulative layer 50 located on the inside surface ofcoilheads 36 and 38, as well as along the entire axial extent of eachslot 44. As shown, a conductive layer 52 is applied to insulative layer50 radially inward thereof (i.e., toward shaft 12). Preferably,conductive layer 52 will be in electrical communication with core 32,such as by contact with the inside walls of slots 44. As a result,conductive layer 52 will be desirably grounded. Because core 32 isalready grounded, there is generally no need to provide a conductivelayer on the inner surface of core 32 between adjacent winding slots. Anonconductive top coat 54, such as a known protective paint, may beapplied over conductive layer 52, as well as other exposed surfaces ofstator 30, to provide protection against corrosion and the like.

The various layers of the electrostatic shield arrangement can be easilyseen in FIG. 5. The thickness of insulative layer 50 is selected,depending on the material, to provide the desired resistance. Thedesired resistance in presently preferred embodiments may be generallyat least 10 megohms, which will comply with applicable industrystandards.

Insulative layer 50 may be formed of a cured resin material impregnatedinto the stator windings. For example, various resinous materials, suchas varnishes, epoxys, polyesters or suitable copolymers, may be suitablefor this purpose. Although many embodiments will utilize a thermosetresin, it is believed that resins that are cured by catalytic action mayalso be employed in at least certain applications.

The resin is generally applied in amounts thicker than would otherwisebe the case in order to yield the desired layer thickness. For example,a glass-filled thixotropic epoxy resin may be used due to its tendencyto apply in relatively thick coats. Such a resin is available, forexample, from P.D. George Co. of St. Louis, Mo. In this case, thethickness “a” will preferably be greater than approximately 0.012inches.

As will be appreciated, conductive layer 52 is preferably formed of anonmagnetic material to prevent adverse effects on the magneticproperties of the machine. Conductive layer 52 may be applied toinsulative layer 50 by spraying or brushing, such as by application of aconductive paint. Paints suitable for this purpose which are copperfilled and conductive when dried in a film are available from SpraylatCorporation of Chicago, Ill. In a known application, such paints havebeen used in the past to treat fan screens to reduce static buildup.Generally, the thickness “b” of conductive layer 52 will be greater thanapproximately 0.001 inches.

As a particular advantage, the electrostatic shield arrangementdescribed above is readily compatible with mass production techniques.For example, in the production of many mass-produced motors, the statoris already dipped in varnish to “set” and protect the stator windings.The present invention contemplates that the resin material used for thispurpose is selected and applied to achieve the desired properties.

Toward this end, FIG. 6 illustrates exemplary processing steps in whicha stator core is first supplied having conductive windings situatedthereon. As indicated at 56, the core is subjected to an impregnationprocedure to impregnate the conductive windings with the selected resin.With a thick resin, a vacuum impregnation (VPI) procedure is especiallypreferred to fully impregnate the conductive windings. In addition toproviding an insulative layer on the surface of the winding group asnoted above, an impregnated resin will displace air between adjacentwires which could otherwise produce undesirable corona discharge.

The resin is then partially (“soft”) cured, as indicated at 58, suchthat the resin is less than completely set. For example, a thermosetresin may be removed from the curing oven prematurely. As indicated at60, the stator core may then be subjected to one or more additionalresin impregnations if a desired thickness for the insulative layer wasnot achieved in the initial impregnation. Each subsequent impregnationis then soft cured, as indicated 62.

After the final impregnation, the inner diameter of the stator core ironmay be cleaned as indicated at 64. This step ensures that substantiallyall resin is removed from the inner surface of the stator iron betweenthe winding slots. Resin will remain in the slots, radially inward ofthe windings, to provide the desired insulative layer.

As indicated at 66, conductive paint may then be applied to thepartially cured resin. Next, as indicated at 68, the paint solvent isallowed to evaporate, or is otherwise removed. The resin is then finishcured, as indicated at 70, until it is fully set. Application of thepaint before the resin is fully cured is believed to achieve a moreintegral bonding of the conductive and insulative layers. Finally, theprotective top coat is applied, as indicated at 72.

It can thus be seen that the present invention provideselectromechanical machines having novel electrostatic shieldarrangements. While preferred embodiments of the invention have beenshown and described, modifications and variations may be made thereto bythose of ordinary skill in the art without departing from the spirit andscope of the present invention, which is more particularly set forth inthe appended claims. In addition, it should be understood that aspectsof the various embodiments may be interchanged both in whole or in part.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tobe limitative of the invention so further described in such appendedclaims.

What is claimed is:
 1. An electromechanical machine comprising: a statorhaving an inner surface and having conductive windings located in aplurality of parallel, axially extending winding slots defined in amagnetically permeable core; a rotor disposed axially within saidstator; an electrostatic shield comprising a conductive layer disposedon an inner surface of said stator at least opposite said rotor andelectrically insulated from said conductive windings of the stator; andan insulative layer disposed over substantially the entire inner surfaceof the stator and said electrostatic shield between said conductivewindings of said stator and said rotor.
 2. An electromechanical machineas set forth in claim 1, further comprising an insulative layer having aconformal structure characteristic of a resin that had been applied tosaid stator in an uncured state after said conductive windings wereplaced in said winding slots and subsequently cured to yield apredetermined layer thickness between said conductive windings and saidrotor.
 3. An electromechanical machine as set forth in claim 2, whereinsaid cured resin is a glass-filled thermoset resin.
 4. Anelectromechanical machine as set forth in claim 2, wherein saidpredetermined thickness of said insulative layer is at leastapproximately 0.012 inches.
 5. An electromechanical machine as set forthin claim 1, wherein said conductive layer comprises a conductive paintbonded to said stator.
 6. An electromechanical machine as set forth inclaim 1, wherein said conductive layer is in electrical communicationwith a magnetically permeable core and is grounded thereby.
 7. Anelectromechanical machine as set forth in claim 2, wherein said curedresin substantially entirely impregnates said conductive windings ofsaid stator.
 8. An electromechanical machine comprising: a fixed statorhaving conductive windings located in a plurality of parallel,axially-extending winding slots defined about an inner surface of amagnetically permeable core, said stator further comprising first andsecond coilheads located at opposite axial ends of said magneticallypermeable core; each of said winding slots including an insulative topliner located radially inward of said conductive windings locatedtherein; a movable rotor located radially inward of said stator; anelectrostatic shield arrangement being formed by an insulative layer ofresin material covered by a conductive layer located radially inwardthereof; said insulative layer and said conductive layer beingconformally applied to said stator so as to be located in said windingslots radially inward of said respective top liner and an inside surfaceof said first and second coilheads to interpose said conductive windingsand said rotor; and an insulating, protective top coat applied over saidconductive layer on an inner surface of said stator.
 9. Anelectromechanical machine as set forth in claim 8, wherein saidconductive layer of said shield arrangement comprises a nonmagneticconductive material located radially inward of said insulative layer.10. An electromechanical machine as set forth in claim 9, wherein saidconductive layer is in electrical communication with said magneticallypermeable core and is grounded thereby.
 11. An electromechanical machineas set forth in claim 10, wherein said conductive layer of said shieldarrangement comprises a metallic paint applied to a surface of saidinsulative layer.
 12. An electromechanical machine as set forth in claim11, wherein said metallic paint comprises a copper paint.
 13. Anelectromechanical machine as set forth in claim 8, wherein saidinsulative layer of said shield arrangement comprises a glass-filledthermoset resin applied to said conductive windings to a predeterminedthickness.
 14. An electromechanical machine as set forth in claim 13,wherein said thermoset resin substantially entirely impregnates saidconductive windings of said stator.
 15. An electromechanical machinecomprising: a fixed stator having conductive windings located in aplurality of parallel, axially extending winding slots defined in amagnetically permeable core; a movable rotor operative to have amagnetic flux induced therein by excitation of said conductive windingsof said stator; an insulative layer having a conformal structurecharacteristic of a resin that had been applied to said stator in anuncured state after said conductive windings were placed in said windingslots and subsequently cured to yield a predetermined layer thicknessbetween said conductive windings and said rotor; and a conductive layerof metallic paint bonded to said insulative layer and thereby separatedfrom said conductive windings, said insulative layer and said conductivelayer thereby forming an electrostatic shield arrangement interposingsaid conductive windings and said rotor; and an insulating, protectivetop coat applied over said conductive layer on an inner surface of saidstator.
 16. An electromechanical machine as set forth in claim 15,wherein said conductive layer is in electrical communication with saidmagnetically permeable core and is grounded thereby.
 17. Anelectromechanical machine as set forth in claim 15, wherein saidmetallic paint comprises a copper paint.
 18. An electromechanicalmachine as set forth in claim 15, wherein said cured resin substantiallyentirely impregnates said conductive windings of said stator.
 19. Anelectromechanical machine as set forth in claim 15, wherein said curedresin is a glass-filled thermoset resin.
 20. An electromechanicalmachine as set forth in claim 19, wherein said predetermined thicknessof said insulative layer is at least approximately 0.012 inches.